Process plants typically include numerous control loops networked together to produce a product. A rotary valve is often an important element in a control loop. The rotary valve may manipulate a flowing fluid, such as gas, steam, water, or chemical compounds to keep process variables as close as possible to a desired set point. The rotary valve, sometimes referred to as a valve or rotary valve assembly, typically includes a valve body, internal trim parts, an actuator (e.g., an electric actuator, a hydraulic actuator, etc.) to provide the motive power to operate (e.g., open or close) the valve, and a variety of additional valve accessories, such as positioners, transducers, supply pressure regulators, manual operators, etc.
Electric actuators often employ a motor operatively coupled to a flow control or closure member (e.g., a valve disk) of a valve (e.g., a rotary valve, a control valve, a throttling valve, etc.) via a drive system (e.g., one or more gears). During operation, when electric power is supplied to the motor, the electric actuator rotates the flow control member between a closed position and an open position to regulate fluid flowing through the valve. When the valve is closed, the fluid flow control member is typically configured to sealingly engage a valve seat disposed within the fluid flow path to prevent the flow of fluid between an inlet and an outlet of the valve.
When the valve is in the closed position and electric power is provided to the motor, the motor typically provides sufficient seat load to the fluid flow control member to ensure that the fluid flow control member is in sealing engagement with the seat of the valve. When electric power is removed from the motor, the drive system may be designed to maintain the position of the fluid flow control member relative to the valve seat. However, forces from fluid flow through the valve can be conveyed to the motor when the actuator is in position. These forces may be sufficient to backdrive the motor and, thus, may result in an unwanted position change of the fluid flow control member with unknown effects on the control loop.